The A cup is the smallest size class of brassieres for adult women. Just how big an A cup is varies on both sides of the Atlantic. Wherever women live, however, knowing the approximate dimensions of an A cup is important for fit, comfort and fashionable appearance.

Precisely How Big an A Cup Is

Cup size is measured by the difference between chest measurement below the breast (let’s call this “measurement 1”) and that around the fullest portion of the breast itself (“measurement 2”, for convenience). In the United Kingdom, an A cup can be as little 0.1 inch and up to 1 inch more than chest size. By United States standards, breast size being 0.6 to 1.5 inches more than chest size determines how big an A cup is.

The other method is to measure around the flat part of the chest above the bust (call this “measurement 3”). Once again, measurement 3 less measurement 2 should give women an idea of the bust fullness that will guide their bra purchase. This alternate method works fine when the woman reliably knows from past purchases that she is a cup A or B.

Whichever method is used, how big an A cup is means a bust fullness of just one-tenth of an inch and up to 1.5 inches more than the unclothed rib cage. And when one realizes that even 1.5 inches accounts for both breasts, it stands to reason that A sizes are more common in training bras for prepubescent girls or adult women who are distinctly underendowed.

Taking All Three Measurements Properly to Determine How Big an A Cup Is

It is best to have a friend, another family member or a lingerie section saleslady help. Maintaining the proper position is important. Attempting to take any of the measurements unaided risks overestimating them. For example, the tape might unknowingly not be perfectly horizontal around the back. Or the very act of cocking one’s arms and swinging them around to the back (as when attempting to fasten a bra) flexes the pectoral and deltoid muscles. So you end up with measurements 1 or 3 that are larger than they should be (and a smaller cup size than you deserve).

Proper posture, being comfortable and relaxed without throwing out one’s chest or taking a deep breath are also key to accurately measuring one’s cup size.

Taking these measurements unclothed is the most accurate way for seeing how big an A cup is. In modest cultures, one may opt to wear a comfortable, unpadded bra. Certainly, the push-up type will not do.

To Sum Up

Getting a handle on just how big an A cup is means subtracting measurements 1 and 3 from measurement 2. As long as the difference, the fullness of one’s bust is no greater than 1.5 inches (in U.S. stores) or 1 inch (in the UK), then one can confidently opt for an A cup on the next purchase.